Frequency-modulated pulse generator



July 6, 1948. B. -TREvoR FREQUENCY MODULATED PULSE GENERATOR 3 Sheets-Sheet 1 Filed Jan. 8, 1944 l INVENTOR n infr/P441 /Pf//a/:

ATTORNEY July 6, 1948. B. TREvoR FREQUENCY MODULATED PULSE GENERATOR 5 Sheets-Sheet 2 Filed Jan. 8, l11.944

ATTORNEY .my 6, 194s.

B. TREvoR 2,444,479

lFREQUENGY MODULATED PULSE GENERATOR Filed Jan. s, 1944 5 shets-sheet 5 Esa INVENTOR fRr/PAM 7435/01?.

stirs nur FREQUENCY-MODULATED PULSE GENERATR Bertram Trevor, Riverhead, N. Y., assgnor to Radio Corporation of America, a corporation of Delaware Application January S, 1944, Serial No. 517,537 7 Claims. (Cl. 179-1715) This invention relates to a pulse generator syspulses may have a length or duration of the order tem having,r means for generating pulses and for of two microseconds, and spaced from one anmodulating the frequency or rate of recurrence other by a time interval of the order of 98 micro'- of the generated pulses in accordance with the seconds. signal modulation. In the embodiment of the invention of Fig, 1,

An object of the present invention is to enable there are provided a pair of oscillators 3 and 6 the generation of pulses whose pulse rate is ad- Whose outputs beat with each other in a frejustable over a desired range, and Whose pulse quency converter 5 to produce a difference frerate can be frequency modulated by the intelliquelley. OSCllatOr 3, by Way of illustration may gence to be conveyed. l0 generate oscillations of 200 kilocycles and is pref- A more specific obje-ct is to provide a system erably non-tunable except for a Vernier adjustfor generating pulses which are short compared ment. Oscillator t. is tunable and may generate to the time interval between them and Whose oscillations in the range from 170 kilocycles to pulse rate is adjustable over a range of kilocycles. 200 kilocycles. depending upon its tuning. Osand having means for modulating the frequency le cillator is provided with a tuning dial calibrated of the pulse rate up to a deviation of plus and in the range from 0-30 kilOCyCles Which S the minus several kilocycles.' range of the difference frequencies obtainable Other objects will appear from a reading 0f from converter 5. the following description which is accompanied A coupling tube 4 Serves to isolate the two 0sbyadrawingwherein: cillators 3 and 6 from each other; that is, to Fig, l diagrammatically shows an embodiment prevent interaction. A band pass selective filter of the lpresent invention; and in the output of tube 4 rejects the harmonics Figs. 2a and 2b taken together illustrate the fl0mOsC11at0r3. detailed circuits of the system of Fig. 1. The oscillator' 3 has its output frequency varied Broadly stated, the present invention includes by e frequency 111001111211301' ll the ferm 0f e rea beat frequency oscillator Whose output deteractance tube circuit 2 to Whose'nput, in 011111, S mines the pulse rate. The beat frequency outapplied the audio modulation from lead l. This put varies up and down from a ldesired mid freaudio modulation -may comprise speech covering quency at a rate depending upon the modulation a frequency range from zero to 3 or 5 kilocycles. frequency which is applied to one of the beating The Output from the frequency converter 5 is of oscillators. The other beating oscillator is adsine wave form and has a frequency deviation dejustable in frequency by means of a dial caltermined by the audio input level to the frebrated over the desired beat frequency output quency modulator 2. This frequency deviationis range. The variable beat frequency output confairly linear for deviations of l5 kilocycles from trols a square wave generator of the flip-flop type the mid or unmodulated frequency in the output having two degrees of electrical stability. This of the frequency converter 'but departs slightly square Wave generator changes from one condifrom linearity for higher deviations. It will-thus tion of stability to the other only in response to be evident that the output of converter 5 coma wave of desired polarity. The output pulses prises a beat frequency in the range from zero from the square Wave generator are passed 40 to thirty kilocycles, depending upon the tuning through a dilerentiator which produces short of oscillator 6.

sharp impulses from the edges or slopes of the In order to eliminate the higher oscillator fresquare Wave pulses. Certain Iones of these short' quency components present in the output of the sharp impulses (i. e., those having a desired pofrequency converter 5, there is provided a 10W larity and produced from the starting edge only pass vfilter 'l which passes frequencies only up to of the square pulses) are selected and reshaped thirty kilocycles. The sine wave output from in form, amplified and then utilized. If de-v filter 1 with the radio frequency components re.- sired, these reshaped short impulses, which are moved is then passed on to a square Wave genkeyed direct current in character, can control erator 9 through a coupling tube 8. Coupling a radio frequency generator to produce corretube 8 provides a little gain for the output from spondingly short, spaced pulses of radio frequency filter 'l and serves to isolate the square Wave energy. The spacing and duration of the short generator 9 from the filter l, Square Wave genradio frequency pulses are identical with the erator e is a flip-flop multivibrator type of circuit spacing and duration of the short reshaped having two degrees of electrical stability. This pulses. By Way of example only, the final output square Wave Vgenerator remains in one condition of stability or the other until it is changed by an input wave. The output of generator 9 comprises direct current pulses 20 which may, if desired, be substantially 50% mark, and its pulse rate varies with and is determined by the beat frequency output of the frequency converter 5. Thus, the pulse rate vof the pulses produced by the square wave generator has an instantaneous value controlled by the modulation input frequency applied to the reactance tube modulator 2. Since oscillator 3 is frequency modulated, the same modulation is applied to the square Wave generator and controls its output pulse rate.

An RC dilferentiator IIl serves to differentiate the relatively long output pulses from the square wave generator, and produces short, sharp impulses 2I from the starting and trailing edges of the square wave pulses 20. Only those sharp impulses which are obtained from the starting edges of the longer square Wave pulses 20 are used to control a coupling tube and limiter II to produce short direct current pulses 22 each having a duration of about two microseconds. Apparatus Il reshapes the short impulsesso to speak, so that they have a flat top and steep vertical sides.

l A powernamplier I2 is provided to increase the voltage of pulses 22 from a value of, let us say, 250 volts to a value near 1,000 volts, depending upon the anode voltage of the amplifier. The pulses in the output of amplifier I2 are indicated at 23 and are of negative polarity. In order to obtain pulses of a positive polarity, these pulses 23 are passed through a pulse transformer I3. The output pulses from transformer I3 are shown at'Zli. vThe spacing and duration of the pulses 24 from pulse transformer I3 are identical with the spacing and duration of the pulses 22 from the coupling tube and limiter Il.

The direct current positive pulses 24 carrying the signal modulation may beutilized to operate any desired type of circuit, such for example as a radio frequency generator, in order to produce correspondingly short and spaced pulses of radio frequency energy which in turn can be radiated from an antenna, if desired.

Figs. 2a and 2b, taken together, show the details of a circuit arrangement following the principles of Fig. 1. The same parts in both figures have been labeled with the same reference numerals.

Referring to Figs. 2a `and 2b together, the audio input waves are applied through leads I to the primary of the transformer T1. The secondary of the transformer T1 supplies potentials of oppositephase to the No. 3 grids of a pair of tubes comprising the reactance tube modulator 2. This modulator is a push-pull reactance tube circuit the anodes of which are directly connected together in order to bein parallel relation for the Output currents. A push-pull modulator is employed instead of .a single reactance tube circuit in order to compensate for variations in line voltage and to reduce unwanted modulations. It should be noted that the two reactance tubes are of opposite sign so that they combine in phase.

The oscillator 3 produces a relatively fixed frequency but is provided with a Vernier adjustment in the form of a variable condenser C. The oscillator 6 is supplied with a dial 30 which is calibrated in kilocycles in the range from zero to thirty kilocycles. Dial 30 islinked to a variable .condenser CI in order to enable adjustment of the' oscillator 6 anywhere in the range from to 200 kilocycles. The output of the oscillator 3 is passed through the coupling tube 4 and the band pass transformer selective circuit in the output of tube 4 to the No. 3 grid of the frequency converter 5, while the output of the tunable oscillator 6 is coupled to the rst grid of the frequency converter 5. A band pass transformer in the output of coupling tube removes any harmonics of the oscillator 3 which may be present therein. The output of the frequency converter 5 Iafter passing through the low pass filter 'I is impressed upon the grid of the coupling tube when the switch 3i is thrown to the left.

The output of the coupling tube 8 is fed to the square wave generator 9, the latter comprising a pair of vacuum tubes whose grids and anodes are resistively interconnected to form a flip-flop circuit having two degrees of electrical stability. This square wave generator operates at a frequency determined by the sine wave frequency applied thereto from the coupling tube 8. In the operation of the square wave generator Si, one tube always passes current while the other is non-conductive and these current passing conditions are reversed upon the application of a wave of suitable polarity. Thus, the square wave generator will remain in one condition of stability until this condition is reversed by the external drive from the coupling tube 8. The output pulses from the square wave generator are relatively long and may, if desired, be substantially 50% marking pulses. The frequency of the pulses from the generator 9 is determined by the beat frequency (difference frequency) of the oscillators 3 and `li and the instantaneous Value of this difference frequency is controlled by the modulation. Putting it in other words, the output pulse rate from the square wave generator depends upon the modulation applied to the frequency modulated oscillator 3. y

The output pulses from the square wave generator 9 are differentiated by the condenser-resistor combination labeled Ill, and the sharp impulses produced by the diiferentiator are applied to the first grid of the coupling tube and limiter Il. Coupling tube II is operated with zero grid bias (the tube passes current) so that only the negative grid pulses produce an effect, the positive grid pulses being limited off. As a result, only positive pulses appear in the anode circuit of this tube. This is because the tube II normally passes current and only the negative grid pulses momentarily bias the tube II to cut off. The output from apparatus Il is in the form of extremely short pulses of about two microseconds duration or so, and these short pulses are applied to the grid of power amplifier I2. Amplifier I2 is normally biased beyond cut-off and passes current momentarily during the time the positive pulses are applied thereto from the coupling tube II. The anode of amplifier II may be operated with as much as 1500 volts applied to it. This tube serves not only as an amplifier but also as a limiter to square up the pulses applied to its grid by limiting on the peaks. The output of amplier I2, which is now in the form of short negative pulses, is passed through pulse transformer I3 in order to produce output pulses of positive polarity therefrom. The negative pulses appearing in the anode circuit of the amplifier l2 are thus reversed in polarity by the transformer I3. A high voltage output, therefore, appears at the jack 32. The rectifier tube 33 across the secondary of the pulse transformer I3 serves to remove the negative pulse appearing in the transamiga-791 former output. The heaterk of` this tube 33` isV connected to a high'voltage point in order to reduce thepotential appearing between'the heater,A and the cathode;

Switch 3lat the input of the couplingtube 8 is provided so that an external: oscillator may be used to operate the pulsing circuits if for! any reason it is not desired to use theinternal beat` frequency oscillator which comprises the two oscillators 3 and 6 and the frequency con-- verterE. If an' external beat frequency oscil lator is employed, however, it is not possible to frequency modulate the pulses. Theuse ofthis switchy enables a degree of flexibility in the usez ofthe equipment. Jack 34 inthe input circuit-A ofthe coupling tube 8v enables a pair of headphones-to be inserted in thev circuit in order to1 listen to the -audio beatxfrequency. By-ut'ilizing;

suclra pair of headphones, itis a relatively simple;

matter to set the twooscillators Sand 6 to zeroA beat with each other when the switch 3| is con nected to the low pass filter 1.- The `jack 3'5fin, the cathode circuit of thel power amplifier I2l enables obtaining a low impedance sourcer of pos-` itive pulses from theI cathode resistor in the amplifier circuit I2. The jack 36 connectedvk tothe-anode of the coupling tube Ilv serves as a medium voltage source of direct current positive pulses whose pulse rate is modulated by the audio input.

In setting upv the systemr of Figs. 2a and 2b, the dial 3i! of the tunable oscillator 6 is first set to read zero, in which case both the oscillators 6 and 3 will generate 200 kilocycles oscillations and thus provide zero beat. In order to achieve this accurately, the zero adjustment condenser C of oscillator 3 is also adjusted so that this-zero beat? occurs. Since the dial 39 is directly calibrated to read the beat frequency in the range from nero to thirty'kilocycles, the attendant need only't'urnthe dial so as to adjust the oscillator 6to that position which will give a desired beat frequency output. As an example, if it is desired to obtain aten kilocycle beat frequency output, it is only necessary-to move dial 3l) to number I0', asa result ofwhich the oscillator 6 will produce oscillations of- 190- kilocycles, and theoutput of the bea-t frequency oscillator will be ten kilocycles, which is the difference betweenthe 200 kilocycles produced by fixed oscillator 3 and the i90-kilocycles produced by the tun-able oscillator 6. Now, by modulating the oscillator 3 in accordance with the signal impulse applied to the reactance tube circuit, there will be obtained a beat frequency output which varies up and down from ten kilocycles at a rate depending upon the modulation frequency.

It should be understood that the invention is not limited to any particular range of beat frequencies, or to any particular frequencies of operation of the oscillators 3 and 6 set forth in the foregoing description, inasmuch as the oscillators 3 and 6 may be designed to produce other frequencies and provide a beat frequency output in a range considerably higher than zero to thirty kilocycles, such for example as a range from Zero to 100 kilocycles or so. The invention has application in a pulse frequency signal generator and is not limited for use in a communication system.

What is claimed is:

1. A frequency modulated pulse generator systern comprising two oscillators producing oscillations of different frequencies, means for modulating the frequency of one of said oscillators in accordance with the signal modulation, a freputs -offboth-oscil-lators for producing a beat fre-y quency converter coupled-to the outputs of said two'- oscillators for producing a difference frequency, aA coupling tube-in the output ofone of1 saidv oscillators for preventing interaction betweensaidoscillators, a lowl pass lter in the output of said converter, and a square wave generator having twodegrees of velectrical stability coupled'A to said-*filter-and under control of the output of -said'ifrequency converter.

2. .ifrequency modulated pulse generator systemL comprising two oscillators producing oscillations-of different frequencies, a-reactance tube modulatorcircuit including a source of audio input' for modulating the frequency of one of said oscillators, means for tuning the other oscillator over arange' of frequencies of the order of kilocycles, affrequency converter coupled to the outquency which isvariable in dependence upon the modulation frequency, a coupling tube in the output of one of said oscillators for preventing interactionbetween said oscillators, a pulse generatorcoupled to the output of saidfrequency converter for producing direct current pulses whose pulse rate is determined by the beat frequency; anda ldifferentiator circuit in the output' ofI said pulse generator for producing short pulses from the edges of said direct current pulses.

3. Av frequency modulated pulse generator system' comprising two oscillators producing oscillations of different frequencies, a reactance tube modulatorcircuit including a source of audio input formodulating` the frequencyof one ofsaid oscillators, means for tuning the other oscillator over arangeof frequencies of the order of kilocycles, a frequency converter coupled to the outputs ofr both oscillators for producing a beat frequency -Which-is variable in dependence upon the modulationfrequency, a coupling tube in they output of one of said oscillators for preventing interaction `between said-oscillators, a pulse generator coupled to the output vof said frequency converter for producing direct current pulses whose pulserate is determinedby the beat frequency, a diiferentiator inthe output of said pulse generator'- for producing short and sharp pulses from-'the edges of said direct current pulses; and an electron discharge device for convertingthose` short sharp pulses which are of a l predetermined polarityinto short direct current pulses having relatively long timev intervals between them.

4.. A' frequency modulated pulse generator systern comprising two oscillators producing oscil- Y lations of different frequencies, a rcactance tube 'modulator circuit including a source of audio input for modulating the frequency of one of said oscillators, means for tuning the other oscillator over a range of frequencies of the order of kilocycles, a frequency converter coupled to the outputs of both oscillators for producing a, beat frequency which is variable in dependence upon the modulation frequency, a coupling tube in the output of one of said oscillators f-or preventing interaction between said oscillators, a pulse generator coupled to the output of said frequency converter for producing direct current pulses whose pulse rate is determined by the beat frequency, a differentiator in the output of said pulse generator for producing short and sharp pulses from the edges of said direct current pulses, an electron discharge device for converting those short sharp pulses which are of a predetermined polarity into short direct current pulses having relatively long time intervals between them, means for substan- 5. A frequency modulated pulse generator sys v tem comprising two oscillators producing oscillations of different frequencies, means for modulating the frequency of one of said oscillators in accordance with the signal modulation, a frequency converter coupled to the outputs of said two oscillators for producinga difference frequency, a square wave generator having two degrecs of electrical stability under control of the output of said frequency converter, a differentiator in the output of said square wave generator for producing sharp pulses from the starting edges of pulses produced by said square wave generator, and means for reshaping said sharp pulses to square pulses which are considerably shorter than the time intervais between them.

6. A frequency modulated pulse generator system comprising two oscillators producing ,oscillations of different frequencies, means for modulating the frequency of one of said oscillators in accordance with the signal modulation, a frequency converter coupled to the outputs of said two oscillators for producing a difference frequency, a coupling tube and a selective circuit in the output of one of said oscillators for preventing interaction between said oscillators and for rejecting harmonics of said last one oscillator, a square wave generator having two degrees of electrical stability under control of the output of said frequency converter, to thereby produce direct current pulses of substantially 50% length, a diferentiator in the output of said pulse generator for producing very short pulses from the starting and trailing edges of said direct current v pulses, and means responsive to those very short pulses which are derived from only one of said edges for producing direct current pulses which are appreciably short compared to the time intervals between them.

i 7. A frequency modulated pulse generator system comprising two oscillators producing oscillations of different frequencies, a reactance tube modulator circuit including a source of audio input for modulating the frequency of one of said oscillators, means for tuning the other oscillator` over a range of frequencies of the order of kilocycles, a frequency converter coupled to the outputs of both oscillators for producing a beat fre'- quency which is variable in dependence upon the modulation frequency, a coupling tube in the output of one of said oscillators for preventing interaction between said oscillators, a pulse generator coupled to the output of said frequency converter for producing direct current pulses whose pulse rate is determined by the beat frequency, a switch between said pulse generator and said frequency converter, said switch having a pair of contacts and an arm adapted to engage either one of said contacts, a connection from one of said contacts to said frequency converter, a connection from the other of said contacts to a third oscillator, and a connection from said arm to said pulse generator, a jack in the output circuit of said frequency converter for enabling observation of the beat frequency, a differentiator in the output of said pulse generator for producing short and sharp pulses from the edges of said direct current pulses, an electron discharge device for converting those short sharp pulses which are of a predetermined polarity into short direct current pulses having relatively long time intervals between them, means for substantially increasing the voltage of said short direct current pulses and for limiting the same, a jack in circuit with said means for deriving direct current pulses of a predetermined' polarity whose pulse rate is modulated by said audio input, and a radio frequency oscillator under control of the output of said last pulses for producing correspondingly short, spaced pulses of radio frequency energy.

BERTRAM TREVOR.

REFERENCES CITED The following references are of record in the file of this patent:

UNlTED STATES PATENTS Number Name Date 793,649 Fassenden July 4, 1905 1,629,685 Ditcham May 24,1927 1,717,630 Schaffer -1 June 18, 1929 2,027,975 Hansell Jan. 14, 1936 2,061,734 Kell Nov. 24, 1936 2,072,962 Plebanski Mar. 9, 1937 2,113,214 Luck 1 Apr. 5, 1938 2,266,401 Reeves Dec. 16, 1941 2,280,707 Kell Apr. 21 1942 2,303,444 Evans Dec. 1, 1942 FOREIGN PA'IENTS Number Country Date 118,054 Australia Feb. 3, 1944 

